Changes in intracellular ionized calcium are thought to be responsible for light-adaptation of the ventral photoreceptor cells of Limulus polyphemus. I will use Ca ion selective microelectrodes, which I have previously shown suitable for detecting steady-state sub-micromolar amounts of Ca in Aplysia R2 neurons, to measure Limulus photoreceptor Ca levels under the following conditions: 1) Dark-adaptation 2) illumination and 3) pressure injection of Ca-EGTA buffers. Ca changes have previously been determined qualitatively by others with the photoprotein aequorin and more quantitatively with the metallochromic indicator dye, arsenazo III. Arsenazo III offers the disadvantage of being extremely sensitive to pH, Mg and ionic strength changes as well as acting as a Ca buffer as discussed in the Background Rationale and Specific Experiments f, of this grant. By accurately knowing the concentration of Ca, it will aid toward developing a light-adaptation mechanism which will yield insight into the basis of the photoresponse.